A database transaction in MarkLogic Server is only committed when the query
execution is complete. Phrased another way, an update transaction will see a
view of the database that does not reflect any updates made by that transaction.
By using a semicolon, you're putting multiple transactions in a single module
file. This is a very handy feature, but, due to the overhead involved in
opening multiple transactions and complexities of managing side effects, it's
generally best to avoid using semicolons unless necessary. In this example,
using a semicolon would provide a window for the document to change from the
update value. If another query was executed at the same time updating /demo010
to <b>data2</b>, the value returned by the semicolon form would be determined
by whichever query won the race.
I'd suggest doing something like:
let $new := <a>data</a>
return (xdmp:document-insert("/demo010", $new), $new)
This approach works because $new will only actually be returned if the insert
is successful; otherwise, the query will return an error.
James
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of G. Ken Holman
Sent: Saturday, December 05, 2009 10:08 AM
To: [email protected]
Subject: Re: [MarkLogic Dev General] Not able to retrive content from uri
Hi again, Soumadri!
At 2009-12-05 18:49 +0530, Chowdhury, Soumadri wrote:
>I have written two consecutive FLOWR expressions like the following,
>
>let $a1 := "" return (xdmp:document-insert("/demo010",<a>data</a>)),
>let $a2 := "" return (fn:doc("/demo010"))
>
>The problem is the 2nd line returns an empty sequence, instead of
>the content of the uri "/demo010"
>
>How to resolve this problem? Any idea?
I learned the answer recently: it has to do with database
transactions. Certain database transactions are not committed to the
database until the query is complete.
As a test I formally declared the module to be using the Mark Logic
version of the language:
xquery version "1.0-ml";
let $a1 := "" return (xdmp:document-insert("/demo010",<a>data</a>)),
let $a2 := "" return (fn:doc("/demo010"))
... which didn't work as yours didn't work and then changed it to:
xquery version "1.0-ml";
let $a1 := "" return (xdmp:document-insert("/demo010",<a>data</a>));
let $a2 := "" return (fn:doc("/demo010"))
... which worked fine. Note the change of the comma to a semi-colon
at the end of the first assignment.
This is using the Mark Logic extension to XQuery of ending a query
with a semi-colon which allows one to put multiple queries in a
single document. The first query ends at the semi-colon and the
database does its business of committing all of the changes. The
second query is then acting on a complete database with your more
recent addition and finds it.
Since we are now dealing with complete queries instead of portions of
a FLWOR expression, your test can be rewritten as follows:
xquery version "1.0-ml";
xdmp:document-insert("/demo010",<a>data</a>);
fn:doc("/demo010")
I haven't yet learned *when* queries need to be flushed/committed to
the database, and I hope someone on the list can give general
guidelines ... but this is how I recently resolved a related Mark
Logic database issue.
Perhaps is there a formal request one can make in a query to commit
any uncommitted transactions?
I hope this helps.
. . . . . . . . . . . . Ken
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